The invention relates to an adhesion promoter between an oxide ceramic, which has been synthetically produced or is a mixture of natural minerals and consists of one or several metal oxides, in particular zirconium oxide, aluminum oxide or spinel ceramic, and a veneer material, such as silicate ceramic, veneer composites or veneer plastic, that is to be applied onto said oxide ceramic, in particular for dental purposes.
The invention also relates to a method for the use of the adhesion promoter to produce such a composite structure, and to a kit for the production and application of the adhesion promoter.
It is known that oxide ceramics are used to produce dental crowns and bridges having a high load-bearing capacity. For the aesthetic design of such prosthetic frames it is required to apply a tooth-shaded dental ceramic, hereinafter referred to as veneer ceramic (in form of silicate ceramic or also designated as feldspar ceramic or glass ceramic, produced from the main materials feldspar and quartz) on the relatively opaque oxide ceramic surface. Likewise, the veneering material can be a tooth-shaded veneer composite or veneer plastic. In the first step of the procedure, the frame is milled out of a pre-sintered oxide ceramic block by means of diamond-coated instruments. The volume of said frame is about 20% larger than the frame that is to be veneered later. In the subsequent sintering operation (1250° C. to 1600° C.) the frame shrinks to the correct fit. Then, the veneer ceramic is applied to this pre-manufactured and now densely sintered oxide ceramic part and sintered, too (850° C. to 1000° C.). In this process it is intended that the expansion coefficients of the two ceramics are as identical as possible. From a technological point of view the expansion coefficient of the veneer ceramic is slightly smaller than the one of the frame ceramic so that the veneer ceramic is shrunk on the oxide ceramic in a mechanical bond during the sintering and then following cooling processes (e.g. Eichner, Kappert: Dental materials and their processing, Vol. 2, Materials and their clinical processing, Thieme publishing house).
Due to the extreme conditions in the oral environment, the permanent moisture and temperature changes as well as the mechanical load, high tensions are caused exactly at the boundary of both ceramics and as a result the veneer ceramic can directly chip off the surface of the oxide ceramic or the tensions are led into the veneer ceramic so that internal tensions can be produced in it and cause cohesive fractures and thus the chipping in the ceramic.
Because of these numerous problems experts tried to increase the bond strength by a silicate layer on the oxide ceramic and described several methods.
U.S. Pat. No. 4,364,731 A reveals a method in which a layer of silicon dioxide is applied by using a high-frequency magnetron-sputter arrangement.
Another known method (DE 34 03 894 C1) is the application of a silicate layer in a flame hydrolysis process of tetraethoxysilane.
Furthermore, DD 276 453 describes a method in which a silicate chromium oxide layer is applied by a sol-gel solution and then strengthened in a subsequent tempering process (320° C., 2-8 min).
DE 38 02 043 C1 shows a method in which the silicate layer is achieved by a corundum blasting process. Here, a certain amount of silicon dioxide with a mean particle size of <5 μm is added to the blasting corundum. In the impact area of the corundum particles energy densities are locally developed that are sufficient to fuse the fine silicate particles on the surface. All aforementioned efforts are characterized by complex and cost-intensive so equipments and procedures without achieving a real quality leap in the increase of the bond strength and the suppression of the mentioned chipping.